On the twitch page for the modpack there is a gear icon, click that and look for "open folder". Click that, once opened select the folder labeled "Mods". Then download the mod you want and drag it into that folder, also add the related dependencies. Hope this helps!
There two pages might help you with third party mods. But I have to say they aren't really consitent with third party mods, I have a modpack with a few third party mods which are all on the list but it just got rejected 3 times in a row for no reason and I have succesfully uploaded it about 5 times.